Garden Soils for An Alpine Rockery Garden
Garden Soils for Any Garden
There are many factors determining the most suitable type of gardenfor an alpine rockery garden.
Where the rock garden is started from scratch, you should choose the soil mix best suited to the needs of the plants you want to grow. Buildingand sink gardens are simple ways of achieving this.
If the site already exists, it may well be necessary to introduce a soil mixture nearer to the requirements of the plants desired.
Lawns, pavements and rock gardens built on level ground are amenable to such changes but if the site is on a slope, it will be difficult to prevent the erosion of one type of soil into another.
If an acid soil is being introduced into a flat limestone garden, gently mound the existing soil upwards towards the centre of the site. Use a 1000 gauge black plastic liner to cover an area slightly larger than the proposed extent of the site including the mound. Then lay the stone on top of the plastic in the normal way.
This will prevent stagnation andproblems, as well as stopping lime reaching the introduced acid soil.
If a lime soil is being built over an already acid garden, there is no need for mounding or for a liner because lime will not do much harm to acid soil.
Ingredients of Garden Soils
The soil mixtures already mentioned contain three main ingredients: humus, grit or shingle, and loam. Their proportions mainly depend on the amount of light received in the garden and its local climate.
In a shady spot the mixture should contain up to 50% humus combined with equal parts of grit or shingle, and loam.
In a more sunny and open setting reduce the humus content by about 25% and mix it with equal amounts of the other ingredients. The proportions must also be varied according to the windiness of the site: with higher rainfall and less wind on a site, more shingle or grit mill be needed to give better drainage, and vice versa. Apply these mixtures to the top 30-38cm (12-15”) of soil layer of any ground.
Again, the depth varies according to the natural underlying soils; the better the drainage, the shallower the top layer can be, but it must never be less than 15cm (6”).
Humus and Loam In Garden Soils
Humus can be considered to be peat and leaf mould, with more of the latter in preference.
The quality of loams is very variable, so an explanation of good loam may also help. An ideal loam is the layer of soil immediately below meadow grass; the grass roots give it a fibrous texture and being neither sand nor clay it can be easily crumbled in the hands. Because it contains perennial weed seeds, it must always be sterilized before use.
Grits and Shingles In Garden Soils
Grits and shingles are coarser than sands and vary in texture and size, the grains ranging from 2-30mm (1/12 to 1 1/4”) in diameter.
Their presence in a garden soil mixture enables effective drainage: the coarser the grains, the better the drainage. If the grains are large enough and in sufficient quantity they will hold some moisture underneath, be able to maintain many plants and there will be no danger of over-watering.
Acid or Lime?
Sandstones, which are acidic, can support every type of plant, including lime-lovers. Similarly, acid-loving plants can be grown on many kinds of limestone but not those which release free lime into the soil, as it will inhibit their growth.
An excess of lime in the soil will cause the leaves of acid-loving plants to turn yellow, a condition known as ’chlorosis’. This is easily remedied by watering the soil around the bases of the affected plants with a solution made from sequestrene granules dissolved in water.
In general, however, it is far better and easier to grow plants in their natural soil-types. It is relevant here to examine what the words ’acidic’ and ’limey’ mean in practical terms to a rock gardener.
A standard scale, known as the pH scale, is used to measure the relative acidity of many substances including soil. It ranges between 0-14 as follows:
- 1 – 6 indicates acidity;
- 6.5 – 7.5 is neutral;
- 7.5 – 14 indicates alkalinity or, in the case of a soil sample, liminess.
Simple soil-testing kits can be bought in a gardening shop and by following the manufacturers instructions, you can discover the type of soil in your garden, and stock it accordingly.
As a double check, look at the plants growing near where you live, for instance, if Rhododendrons, Azaleas or heathers are thriving, the soil is acidic. Though not a totally reliable test, because variations can occur over very short distances, it will give you a rough idea of your garden soil.